Abnormal mucus clearance is an important contributor to the phenotype of patients with chronic bronchitis resulting from environmental and/or genetic causes. Increases in airway mucus concentration, as the result of reduced airway hydration and increased mucin secretion, appear to represent a unifying theme in both cystic fibrosis (CF) and COPD patients. However, major advances in our knowledge of the fundamental mechanisms involved in regulating mucus clearance are required to elucidate the mechanism by which hyperconcentrated mucus produces disease pathogenesis. We hypothesize mucus dehydration, combined with alterations in mucus biophysical properties by neutrophil elastase (NE) as a result of neutrophilic inflammation, produces adherent mucus that sticks to epithelial cells and produces in a slowing/failure of cilia- and cough-mediated clearance mechanisms. We have developed a novel description of mucus transport system, i.e., the two-gel mucus layer/periciliary layer (PCL) hypothesis that emphasizes the role of the concentration of secreted mucins, i.e., their hydration, in the mucus layer to predict the efficacy of mucus clearance. Based on this model, we hypothesize that normal mucus clearance requires (1) adequate hydration of the airway surface and (2) an absence of strong adhesive interaction between mucus and cell surface. The main goal of this project is to understand how the mucus and PCL layers are maintained in health and how they fail in disease. Hypothesizes tested in three interacting Aims will be used to expand our understanding of the pulmonary clearance system. In Aim 1, we will investigate the role of the PCL in airway defense, building upon our previously published work in the biophysics of this layer. In Aim 2, we perform studies to understand how mucus dehydration and NE alter the osmotic and cohesive properties of the mucus layer. Finally, in Aim 3, we will combine the knowledge gained in Aims 1 and 2 to understand the how the mucus and PCL layers interact to maintain cilia- and cough-mediated mucus clearance, and why they fail in disease.